Zeolites and zeolitic materials constitute a prominent class of chemical compounds with a wide range of applications due to their unique microporous structure. A prominent and well studied example for a zeolitic material is for example zeolite Beta, which is a zeolite having the BEA framework structure containing SiO2 and Al2O3 in its framework and which is considered to be one of the most important nanoporous catalysts with its three-dimensional 12-membered-ring (12 MR) pore/channel system. As such, it has been widely used in petroleum refining and fine chemical industries. A further notable example is Levyne, which displays an LEV-type framework characterized by heptadecahedral cavities to which these zeolites owe their large micropore volume, although the structure only has small eight-membered ring (8 MR) pore openings. The framework density of Levyne is comparable to those of Chabazite (CHA) and Erionite (ERI) having closely related framework structures. Such small pore zeolites are of importance because they exhibit zeolite-specific definite shape selectivity with respect to reactant molecules in catalyst applications. Furthermore, such small pore zeolites having large micropore volumes are attractive due to their large adsorption capacities.
For achieving such microporous frameworks, zeolites are typically synthesized in the presence of templating agents, usually small organic molecules referred to as organotemplates. In the original synthesis of zeolite Beta for example, which was first described in U.S. Pat. No. 3,308,069, the tetraethylammonium cation was used as the structure directing agent. Later on, other structure directing agents found use in the synthesis of zeolite Beta such as dibenzyl-1,4-diazabicyclo[2,2,2]octane in U.S. Pat. No. 4,554,145 or dibenzylmethylammonium in U.S. Pat. No. 4,642,226. Levyne-type zeolites, one the other hand, were prepared using exotic organotemplates as structure directing agents, such as Quinuclidine-based templates, such that their synthesis typically involved high costs. Lower cost alternatives use diethyldimethylammonium hydroxide as a structure directing agent wherein the diethyldimethylammonium cations act as the organotemplate. U.S. Pat. No. 7,264,789 B1, for example discloses a method for preparing LEV-type zeolites which alternatively uses choline and diethyldimethylammonium as organotemplate. Recently, the LEV-type zeolite RUB-50 was reported in Yamamoto et al. Micropor. Mesopor. Mater. 2009, Vol. 128, pp. 150-157, which was synthesized using the diethyldimethylammonium cation as oraganotemplate.
The use of organic template compounds in the synthesis of these zeolitic materials possesses the major disadvantage that the tetraalkylammonium salts and other organic compounds employed therein are expensive fine chemicals. In addition to this, the resulting products inevitably contain the organotemplates which are encapsulated in the zeolitic framework created around them, such that a removal step becomes necessary in order to open the porous volume of the material for actual utilization.
In addition to these drawbacks, complete removal of the organic template compound is often difficult and may normally only be achieved by calcination at higher temperatures, usually at temperatures ranging from 200-930° C. or even higher. This procedure not only greatly increases the production costs since the organic template is destroyed in the process and may not be recycled, it also further increases the production time, results in excess energy consumption, and produces harmful gases and other unwanted waste products. Finally, the harsh thermal treatment ultimately limits the production to thermally stable zeolites, and in particular to high-silica zeolitic materials. Although ion-exchange methods have been developed as an environmentally friendly alternative to calcination for removing the organotemplate, only part of the organic templates may successfully be recycled, the remainder interacting too strongly with the zeolite framework for removal.
Recently, however, it has been discovered that zeolite Beta and Levyne may also be prepared in the absence of the organotemplates, which until then had always been used as structure directing agent. Thus, in Xie et al., Chem. Mater. 2008, 20, pp. 4533-4535, a process for the synthesis of zeolite Beta is shown, in which crystallization of an aluminosilicate gel is conducted using zeolite Beta seed crystals. In WO 2010/146156 A the organotemplate-free synthesis of zeolitic materials having the BEA framework structure, and in particular to the organotemplate-free synthesis of zeolite Beta is described. In Majano et al., Chem. Mater. 2009, 21, pp. 4184-4191, on the other hand, Al-rich zeolite Beta materials having Si/Al ratios as low as 3.9 are discussed which may be obtained from reactions employing seeding in the absence of organic templates. As regards Levyne, a seed-directed synthesis in the absence of organic templates is described in Xie et al., Chem. Commun. 2011, 47, pp. 3945-3947.
Although the seeded synthesis offers several advantages compared to the previous routes employing organotemplates, the organotemplate-free synthesis affords clearly lower yields compared to the latter. As a means of increasing the yield in the organotemplate-free synthesis of zeolite Beta, Majano et al. describes the use of nano-sized zeolite Beta seeds. Although this is reported to improve the yield to some extent, the yields achieved by templated synthetic methodologies may nevertheless not be realized, let alone surpassed. Accordingly, although improvements have been made with respect to avoiding the use of costly organotemplates in the synthesis of zeolitic materials, there is an ongoing need to find an efficient method for the production thereof which is not only environmentally friendly by may also actually compete with the templated methodologies in terms of both the time and costs of production.